The script that you write needs to insure that only matching words are compared for difference. You can place the code that inserts a "+" at the first string difference position into a subroutine that can be called from anywhere within your script. For example:

use strict;
use warnings;

my $string1 = 'amayaM';
my $string2 = 'amayamAn';

my $result = firstStrDiff( $string1, $string2 );

print $result, "\n";

sub firstStrDiff {
    my ( $s1, $s2 ) = @_;

    ( $s1 ^ $s2 ) =~ /[^\x00]/;
    substr( $s2, $-[0], 0 ) = '+' if defined $-[0];
    return $s2;
}
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Output:

amaya+mAn
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This can help you focus on just the remaining code that aligns the words you want send to this subroutine.

Hope this helps!